The present invention generally relates to a resistor network having multiple different or common values in a single device manufactured using a single sacrificial layer.
Resistors are presently fabricated using a number of different methods, depending on the requirements of the circuit in which resistors are to be used. Resistor types such as thin film resistors, thick film resistors, wound wire resistors, molded axial leaded resistors, surface mount resistors and others are known in the art.
Thin film resistors are fabricated by first depositing a resistive material, then a conductor material, onto a substrate. A wide variety of substrate materials can be used, but these materials generally contain an oxygen compound to permit adhesion of the resistive film. Materials used for thin film resistors generally must also be capable of providing adhesion to a substrate. The resistor film forms as single points on the substrate in the vicinity of substrate faults or other irregularities that might have an excess of broken oxygen bonds. The points expand into islands that form continuous films.
This differs from typical thick film resistor fabrication for which the conductor is deposited first onto a substrate, followed by the resistive material. In general, thick film resistors are formed by adding metal oxide particles to glass particles and firing the mixture at a certain temperature and for a predetermined time period sufficient to melt the glass and sinter the oxide particles together. The resulting structure consists of a series of three-dimensional chains of metal oxide particles embedded in a glass matrix. The higher the metal oxide-to-glass ratio, the lower the resistivity.
The drawback of traditional thick and thin film resistor manufacturing is that both processes tend to have many process steps. Another drawback to these methods of manufacture is that they are generally not capable of providing resistor networks having multiple different or common values in a single device. Further, current methods of manufacturing resistors generally result in the leads being located at the periphery of the resistor device. Peripherally located leads often suffer from the problem that these leads can be easily bent, requiring, in certain circumstances, that the resistor be glued in place. The proposed methods are more versatile than existing methods in that a wide range of resistor devices can be built using a single sacrificial layer, and that the resistor devices can be designed without peripherally located leads.
One aspect of the present invention provides methods of making resistors. A method of making resistors in accordance with this aspect of the invention desirably includes the steps of providing a sacrificial layer having a first surface and one or more pads including at least one electrically conductive material disposed over at least one region of said first surface, and depositing a resistive material over the pads and over said first surface of said sacrificial layer to thereby form at least one unit including the resistive material and the pad or pads. Following deposition of the resistive material, at least a portion of the sacrificial layer is removed so as to expose the one or more pads. Most preferably, a plurality of resistors is manufactured simultaneously using a single sacrificial layer. The method further may include separating at least some of said resistors from one another, typically after removing the sacrificial layer. For example, the resistive layer may form a large unit including numerous pads and a unitary resistive layer, and such unit can be subdivided to form individual resistors or resistor networks, each including a portion of the layer and at least one pad, and typically at least two pads.
The step of providing the sacrificial layer and the pads preferably includes depositing the conductive material onto said first surface of said sacrificial layer. In certain preferred embodiments, cavities are provided in the first surface of the sacrificial layer and the step of depositing the conductive material includes depositing at least one conductive material into these cavities. The step of providing said cavities in the first surface of the sacrificial layer desirably includes providing an apertured layer on the first surface and etching said first surface through the apertures in said apertured layer. For example, the apertured layer may be provided by providing a layer of a patternable material such as a photoresist and exposing the patternable material to light or other radiation in a pattern, and then developing the photoresist to form the apertures.
The conductive material of the pads may be deposited through the apertures, so as to form each pad with a bottom flange beneath the apertured layer, within a cavity, a post extending through an aperture, and a top flange overlying the top surface of the apertured layer. The apertured layer may be partially or completely removed, so as to leave the top flanges of the pads elevated above the surface of the sacrificial layer or above the surface of the remaining part of the apertured layer. The resistive material can be applied in a flowable state, as, for example, by molding, calendaring or coating, so that the resistive material encapsulates the top flanges and posts, thus firmly uniting the pads with the resistive material.
In a method according to a further aspect of the invention, the pads may be formed as hollow shells within the cavities of the sacrificial layer. The resistive material may penetrate into the interior spaces within these hollow shells.
A method of making resistors according to a further aspect of the invention desirably includes the steps of providing a sacrificial layer having a first surface and a second surface; depositing resistive material over the first surface of said sacrificial layer so that the resistive material adheres to the sacrificial layer; and selectively removing portions of the sacrificial layer to form one or more pads connected to said resistive material. For example, spots of an etch-resistant material can be applied to the second surface of the sacrificial layer, and the second surface can be exposed to an etchant so that portions of the sacrificial layer are left in place as individual pads where the etch-resistant material was applied.
Methods according to the foregoing aspects of the invention provide efficient manufacturing processes for forming resistors and for forming resistor networks incorporating plural resistances. The resistors and resistor networks can be compact, and can be particularly well-suited to circuit manufacturing techniques such as surface mounting on a printed circuit board or other circuit panel.
Still further aspects of the invention provide resistors and resistor networks.
Because understanding of the present invention can be facilitated by understanding of the disclosure in the aforementioned U.S. Pat. No. 6,001,671 (hereinafter the xe2x80x9c""671 disclosurexe2x80x9d), certain portions of the ""671 disclosure are reproduced hereinbelow.